1. Field of the Invention
The present invention relates to a wide-angle lens system used in a single-lens-reflex (SLR) type electronic digital still camera (hereinafter, a digital SLR camera).
2. Description of the Prior Art
A digital SLR camera has usually employed an imaging device whose picture size is smaller than the picture size of a SLR camera with a silver-halide film (hereinafter, an SLR camera), since it has been difficult to increase the size of an imaging device.
On the other hand, the lens mount of a digital SLR camera is often made common to that of an SLR camera, such as ones disclosed in Japanese Unexamined Patent Publication (JUPP) No. 6-160706 and JUPP No. 2003-29141. Due to this arrangement, many types of photographic lenses for SLR cameras, which have already been available on the market, can be utilized as an exchangeable photographic lens for digital SLR cameras.
However, in the case where a photographic lens for an SLR camera is used with a digital SLR camera, the angle-of-view at the position of the imaging device becomes narrower such as like the so-called trimming. This is because a photographic lens for an SLR camera is designed to correspond to the picture size for a silver-halide film which is larger than the picture size for the imaging device of a digital SLR camera.
Accordingly, an exclusive exchangeable photographic lens for a digital SLR camera having a much wider angle-of-view has been desired. However, as explained, the lens mount of a digital SLR camera has frequently been made common to that of an SLR camera. Therefore the back focal distance equivalent to that of a SLR camera is structurally required, and a wider angle-of-view becomes inevitably necessary to the extent that a bundle of light rays carrying an object image does not fit the picture size for the imaging device. Namely, in order to obtain a wider angle-of-view to meet the picture size for the imaging device, the back focal distance should be shorter to make the angle-of-view wider; however, in this case where the lens mount is commonly used between an SLR camera and a digital SLR camera, the longer back focal distance is required in a digital SLR camera. For example, in the case of an SLR camera with the 35 mm-film format, the back focal distance which is approximately 1.6 times of the radius of an image circle is necessary; while in the case of a digital SLR camera with the imaging device (CCD) of 23.5 mm×15.7 mm size, the back focal distance is required to be approximately 2.5 times of the radius of an image circle.